1. Field of the Invention
The present invention relates to a defect detecting method and apparatus for detecting a defect such as a crack, a void, or a dent which is present in an object to be measured.
2. Description of the Related Art
For example, if the cylinder part or the piston part used in an automobile engine has a defect such as a crack, void, or dent, the cylinder mechanism or the piston mechanism gets defective. To prevent that, it is preferable to detect the part which has the crack, void, or dent in the production line thereof before assembling the automobile engine.
Conventionally, as such defect detecting methods, ultrasonic reflection method, AE (acoustic emission) crack sound detecting method, CCD camera observation method, X ray photographing method, color check method, eddy current method, and so forth have been known.
However, the aforementioned methods have the following problems in handling thereof.
For example, in the ultrasonic reflection method (ultrasonic flaw detecting method), a sensor brings in contact with an object to be measured to detect a defect. Thus, only a defect with which the sensor brings in contact can be detected due to the property of rectilinear propagation of ultrasonic waves. In addition, the wave forms observed vary with reflection due to irregular connecting surface of the sensor and with an angular difference. Thus, this method involves a difficulty for determining the defect.
In the case of the AE method, like the ultrasonic flow detecting method, a sensor brings in contact with an object to be measured. In addition, in this method, only a progressive crack is detectable. Conversely, when the progressive crack is measured, it becomes large.
In the CCD camera observation method, besides a crack and a dent, a stain and a pattern also disadvantageously affect the determination result of the measurement. In addition, in this method, a void called a "cavity" in a casting cannot be determined.
In the X ray photographing method, a defect can be directly observed. However, the adjustment of the X ray amount is difficult and accordingly, the 100% inspection of the objects to be measured cannot be conducted. Thus, this method cannot be used on the production line.
In the eddy current method, an object to be measured should be rotated at a high speed. In addition, to an increase in the sensitivity, it is necessary to approach the sensor to the object to be measured and to equally move them. However, if the surface of the object to be measured is uneven, it is very difficult to measure a defect.